1. Field of the Invention
The present invention relates to a photo detector circuit for use in a photodiode amplifier or the like for amplifying and outputting a light signal received by a photodiode for example.
2. Description of the Related Art
In a reproducing apparatus or a reproducing section of a recording/reproducing apparatus adapted to play an optical disk or a magneto-optical disk for example, there is performed an operation of generating a reproduced RF signal or a servo error signal for execution of tracking servo or focus servo control, on the basis of a light signal derived from a reflected laser beam received by a photo detector of an optical pickup. Normally the signal received by such a photo detector is supplied, after being amplified, to a predetermined function circuit in the following stage.
FIG. 1 is a circuit diagram showing an exemplary photodiode amplifier circuit used to amplify a received light signal obtained by a photodiode which constitutes such a photo detector.
In this diagram, a photodiode PD1 serves as a photo detector, wherein its cathode is connected to a power supply line +Vcc, while its anode is connected to an inverting input of an operational amplifier OP1. It is supposed in this case that a stray capacitance C.sub.1 is existent between the inverting input of the operational amplifier OP1 and the ground.
The operational amplifier OP1 converts the output current (received light signal) i.sub.1 of the photodiode PD1 into a voltage proportional thereto and then amplifies the same. A resistor R.sub.1 is connected between the inverting input of the operational amplifier OP1 and an output terminal. A capacitance denoted by reference numeral C.sub.2 corresponds to a stray capacitance or a capacitor provided to flatten the frequency characteristic of the operational amplifier OP1 by suppressing the high-range peak in the frequency characteristic. Hereinafter the capacitance represented by C.sub.2 will be regarded as a stray capacitance.
In this diagram, an input equated noise v.sub.n1 to the operational amplifier OP1 is shown equivalently as it is inserted between a non-inverting input of the operational amplifier OP1 and the ground.
In the photodiode amplifier circuit of FIG. 1, when an output current i.sub.1 of the photodiode PD1 is obtained, the output voltage v.sub.1 of the operational amplifier OP1 is expressed as follows in a frequency range where the effects of stray capacitances C.sub.1 and C.sub.2 are merely slight. EQU v.sub.1 =-i.sub.1 .multidot.R.sub.1 +v.sub.n1 (1)
FIG. 2 shows a structural example of another photodiode amplifier circuit. The configuration of the photodiode amplifier circuit shown in this diagram is formed by modifying the photodiode amplifier circuit of FIG. 1 into AC coupling type. Since any circuit elements corresponding to those in FIG. 1 are denoted by like reference numerals or symbols, a repeated explanation thereof is omitted. The difference between the two circuits of FIGS. 1 and 2 resides in a portion enclosed with a one-dot chained line.
The configuration of the photodiode amplifier circuit shown in FIG. 2 is constituted by adding a resistor R.sub.2 and a DC blocking capacitor C.sub.3 to the circuit of FIG. 1. The resistor R.sub.2 is inserted between the anode of the photodiode PD1 and the ground, and the DC blocking capacitor C.sub.3 is inserted between the anode of the photodiode PD1 and the inverting input. In this case, therefore, only an AC component alone is supplied as the output current i.sub.1 of the photodiode PD1 to the inverting input of the operational amplifier OP.
In this case, if the resistor R.sub.2 and the DC blocking capacitor C.sub.3 are so set as to be sufficiently greater than the other resistor (R.sub.1) and capacitances (stray capacitances C.sub.1 and C.sub.2), then it follows therefrom that the output voltage v.sub.1 expressed by Eq. (1) can be obtained, in the predetermined signal band, from the operational amplifier OP1 as in the aforementioned photodiode amplifier circuit of FIG. 1.
In playing a magneto-optical disk for example, it is generally customary that, although a detailed explanation is omitted here, a differential detection output relative to the received light signals of two photo detectors is obtained for reproduction of RF signals by reading out from the disk the data recorded in the form of magnetic pits.
FIG. 3 is a circuit diagram of a photodiode amplifier circuit which is employable as a differential detector circuit to execute differential detection with respect to the received light signals of two photo detectors. Any circuit elements corresponding to those in FIG. 2 are denoted by like reference numerals or symbols, and a repeated explanation thereof is omitted.
In this case, the above-described two photo detectors correspond to photodiodes PD1 and PD11.
Since the amplifier circuit in FIG. 3 for amplifying the output current i.sub.1 of the photodiode PD1 is structurally the same as that of FIG. 2, any circuit elements corresponding to those in FIG. 2 are denoted by like reference numerals or symbols, and a repeated explanation thereof is omitted.
Meanwhile another amplifier circuit for amplifying the output current (received light signal) i.sub.11 of the photodiode PD11 comprises an operational amplifier OP1, resistors R.sub.11 and R.sub.12, stray capacitances C.sub.11 and C.sub.12, a DC blocking capacitor C.sub.13, and an input equated noise v.sub.n11. In this circuit, the connection of the component elements is the same as that in the amplifier circuit which comprises an operational amplifier OP1, resistors R.sub.1 and R.sub.2, stray capacitances C.sub.1 and C.sub.2, a DC blocking capacitor C.sub.3, and an input equated noise v.sub.n1 for amplifying the output current (received light signal) i.sub.1 of the photodiode PD1.
The output voltage v.sub.1 of the operational amplifier OP1 is supplied to an inverting input of a differential amplifier OP3, while the output voltage v.sub.11 of the operational amplifier OP11 is supplied to a non-inverting input of the differential amplifier OP3. The differential amplifier OP3 is supposed to have an amplification factor k.
In the above circuit configuration, the output voltage v.sub.1 of the operational amplifier OP1 becomes the same as the one expressed by Eq. (1) when the values of the resistors R.sub.2, R.sub.12 and the DC blocking capacitors C.sub.3, C.sub.13 are sufficiently greater than those of the other capacitors C and resistors R and in a frequency range where the influences of the stray capacitances C.sub.1, C.sub.2, C.sub.11, C.sub.12 are merely slight. Meanwhile the output voltage v.sub.11 of the operational amplifier OP11 is expressed as follows in accordance with Eq. (1). EQU v.sub.11 =-i.sub.11 .multidot.R.sub.11 +v.sub.n11 (2)
The differential amplifier OP3 produces its differential output voltage v.sub.0 which represents the difference between the output voltage v.sub.1 of the operational amplifier OP1 and the output voltage v.sub.11 of the operational amplifier OP11. This differential output voltage v.sub.0 is expressed as EQU v.sub.0 =k(v.sub.11 -v.sub.1) (3)
Supposing now that the circuit shown in FIG. 3 for example is a differential detection circuit of a photo detector adapted for a magneto-optical disk, it is considered that the resistors R.sub.1, R.sub.11, the amplification factor k and the output currents i.sub.1, i.sub.11 mutually have the following relationship. EQU R.sub.1 =R.sub.11 =R EQU k=1 EQU i.sub.1 =-i.sub.11 =i (4)
Further, input equated noises v.sub.n1 and v.sub.n11 from random generation sources can be simplified as EQU v.sub.n11 -v.sub.n1 =.sqroot.2v.sub.n (5)
Therefore, the differential output voltage v.sub.0 obtained from the differential amplifier OP3 in this case can be expressed as EQU v.sub.0 =2i.multidot.R+.sqroot.2v.sub.n (6)
In any of the photodiode amplifier circuits described above with reference to FIGS. 1 to 3, the received light signal of each photodiode is weak, and consequently a satisfactory S/N (signal-to-noise ratio) is required to ensure high-reliability data reproduction.
In case the input equated noise v.sub.n1 or v.sub.n11 to the operational amplifier is dominant over the S/N of the photodiode amplifier circuit, the S/N can be improved by increasing the value of the resistor R.sub.1 or R.sub.11 connected to the operational amplifier. However, since the output band width f.sub.z of the operational amplifier is decided as EQU f.sub.z =1/(2.sqroot..multidot.C.sub.2 .multidot.R.sub.1) (7)
there arises a problem that the band width is narrowed with an increase of the value of the resistor R.sub.1 or R.sub.11. Since the stray capacitance C.sub.2 acts to flatten the frequency characteristic, a limit is existent in decreasing the value thereof. Accordingly, improvement of the S/N by increasing the value of the resistor R.sub.1 or R.sub.11 to maintain a sufficient band width is somewhat restricted, and this means is not considered to be effective.
In order to solve the above problem, there is proposed, by the present applicant, a photodiode amplifier circuit having a configuration of FIG. 4, as exemplified in U.S. application Ser. No. 08/704721 filed on Jan. 31, 1996. The photodiode amplifier circuit of FIG. 4 also is structurally a differential detection circuit which performs differential detection with respect to received light signals of two photo detectors, similarly to the aforementioned amplifier circuit of FIG. 3. Any circuit elements corresponding to those in FIG. 3 are denoted by like reference numerals or symbols, and a repeated explanation thereof is omitted.
In the photodiode amplifier circuit of FIG. 4, a resistor R.sub.4 is inserted in series between a photodiode PD1 and a power supply line +Vcc, and similarly a resistor R.sub.14 is inserted in series between a photodiode PD11 and a power supply line +Vcc. Further a DC blocking capacitor C.sub.4 is inserted between a junction of the resistor R.sub.4 and the cathode of the photodiode PD1, and an inverting input of an operational amplifier OP11. Similarly, a DC blocking capacitor C.sub.14 is inserted between a junction of the resistor R.sub.14 and the cathode of the photodiode PD11, and an inverting input of an operational amplifier OP1.
In such a circuit configuration, output currents i.sub.1 and i.sub.1a, which are equal in quantity but opposite in phase to each other, can be obtained from the anode and cathode of the photodiode PD1. Similarly, output currents i.sub.11 and i.sub.11a, which are equal in quantity but opposite in phase to each other, can be obtained from the anode and cathode of the photodiode PD11.
When the values of the resistors R.sub.2, R.sub.12, R.sub.4, R.sub.14 and the DC blocking capacitors C.sub.3, C.sub.4, C.sub.13, C.sub.14 are sufficiently greater than those of the other capacitances C and resistors R and in a frequency range where the influences of the stray capacitances C.sub.1, C.sub.2, C.sub.11, C.sub.12 are merely slight, the output voltage v.sub.1 of the operational amplifier OP1 is obtained by amplifying, as expressed in Eq. (8) below, the current i.sub.2 produced through addition of the output currents i.sub.1 and i.sub.11a which are delivered respectively from the photodiodes PD1 and PD11 and have an in-phase relationship mutually. EQU v.sub.1 =-i.sub.2 .multidot.R.sub.1 +v.sub.n1 EQU i.sub.2 =i.sub.1 +i.sub.11a (8)
Similarly, the output voltage v.sub.11 of the operational amplifier OP11 is obtained by amplifying, as expressed in Eq. (9) below, the current i.sub..sub.12 produced through addition of the output currents i.sub.11 and i.sub.1a which are delivered from the photodiodes PD11 and PD1 respectively and have an in-phase relationship mutually. EQU v.sub.11 =-i.sub.12 .multidot.R.sub.11 +v.sub.n11 EQU i.sub.12 =i.sub.11 +i.sub.1a (9)
Further, the output voltage v.sub.0 of the differential amplifier OP3, to which the output voltages v.sub.1 and v.sub.11 of the operational amplifiers OP1 and OP11 are supplied, is expressed as EQU v.sub.0 =k(v.sub.11 -v.sub.1) (10)
Assuming th at the circuit shown in FIG. 4 is a differential detection circuit of photo detectors adapted for a magneto-optical disk, the resistors R.sub.1, R.sub.11, amplification factor k and output currents i.sub.11, i.sub.1a, i.sub.11, i.sub.11a can be simplified as EQU R.sub.1 =R.sub.11 =R EQU k=1 EQU i.sub.1 =-i.sub.1a =-i.sub.11 =i.sub.11a =i (11)
Further the input equated noises v.sub.n1 and v.sub.n11, which are random generation sources, can be simplified as Eq. (5) mentioned already. Therefore, the output voltage v.sub.0 of the differential amplifier OP3 in this case is expressed as EQU v.sub.0 =4i.multidot.R+.sqroot.2v.sub.n (12)
Comparing the output voltage v.sub.0 (Eq. (12)) of the differential amplifier OP3 in the photodiode amplifier circuit of FIG. 4 with the output voltage v.sub.0 (Eq. (6)) of the differential amplifier OP3 in the photodiode amplifier circuit of FIG. 3, the signal output obtained in the circuit of FIG. 4 is twice on condition that the noise components included in the output voltages v.sub.0 of the two differential amplifiers OP3 are equal to each other, so that the S/N is also improved twice as compared with the circuit of FIG. 3.
In manufacture of a photo detector, it is generally customary that, regarding a physical structure of semiconductor, at least cathodes of a predetermined number of photodiodes constituting a desired photo detector are formed to be mutually common.
However, in the photodiode amplifier circuit of FIG. 4 for example, the resistors R.sub.4 and R.sub.14 need to be connected respectively to the cathodes of the photodiodes PD1 and PD11, whereby at least two of the plural photodiodes of the photo detector corresponding to PD1 and PD11 should be so formed that the anodes and cathodes thereof are mutually independent.
Also in the photodiode amplifier circuit of FIG. 4, it is necessary to extract the received light signals, which are AC components, from the photodiodes PD1 and PD11 via the DC blocking capacitors C.sub.3, C.sub.13, C.sub.4, C.sub.14, so that the amplifier circuit configuration is limited only to an AC coupling type, and amplification of DC signal is impossible.
Further, in any of the photodiode amplifier circuits described heretofore, it is preferred that the whole structure be shaped into a single chip with dimensional reduction. However, since the capacitance values of the DC blocking capacitors C.sub.3, C.sub.4, C.sub.13, C.sub.14 and the resistance values of the resistors R.sub.2, R.sub.4, R.sub.12, R.sub.14 are great in the circuit of FIG. 4, it is difficult to construct the whole structure into a single chip inclusive of such component elements.